In mobile work machines or agricultural vehicles, it is customary for the hydraulic components to be supplied by separate hydraulic circuits, such as a main hydraulic circuit or a transmission hydraulic circuit. Depending on the design type, the main hydraulic circuit is divided into different circuits or branches in order to supply steering, brake and other hydraulic components with hydraulic fluid.
A transmission hydraulic circuit normally includes both a transmission shift control circuit and a lubricating hydraulic circuit, so that both the transmission shifting and the lubrication and cooling of the bearings and other moving transmission parts are ensured.
Transmission hydraulic circuits are normally supplied by a cheap and simple fixed displacement pumps. However, such a fixed displacement pump must be designed to provide sufficient volumetric flow rate at idle and, as the engine speed increases, increasingly deliver excess unused hydraulic fluid into the tank. This produces unnecessary efficiency losses for the vehicle.
In the case of the main hydraulic circuits, in order to avoid such efficiency losses, variable or adjustable main hydraulic pumps or variable displacement pumps with load-sensing hydraulic systems are used. However, these variable displacement pumps generally have only a limited suction capacity, on account of which charge pumps are used, which supply or charge these variable displacement pumps with low-pressure hydraulic fluid. Charge pumps are usually constructed as fixed displacement pumps with good suction characteristics, so that, even when used in cold surroundings, a sufficient suction pressure is ensured. However, in the use of a charge pump configured as a fixed displacement pump, efficiency losses generally ensue, since the fixed displacement pumps must always deliver somewhat more hydraulic fluid than the variable displacement pump can maximally accept. The rest of the hydraulic fluid is led off to the tank. The efficiency losses rise if the variable displacement pump accepts, for instance, no hydraulic fluid, so that the whole of the hydraulic fluid delivered by the charge pump has to be diverted to the tank.